CVYV is a potyvirus (family Potyviridae) which causes severe damages in cucurbits, such as cucumber, melon, watermelon and zucchini. The virus can be transmitted mechanically and by the natural vector for CVYV, which is the whitefly Bemisia tabaci. The whitefly vector B. tabaci is endemic in many of the world's main watermelon growing areas. CVYV infects both crops in the field, as well as in plastic tunnels.
Resistance against CVYV has been identified in melon and cucumber, but not yet in watermelon. For example WO2010/025747 describes a resistance QTL against CVYV in melon, derived from landrace Cuc6491. WO2011003440 describes CVYV resistance in cucumber.
Symptoms of CVYV in watermelon are sometimes mild leaf chlorosis, but often external symptoms are inconspicuous or not expressed. CVYV infection is then only seen when fruits are found to have developed internal necrosis not seen from the outside, but which makes the fruits unsuitable for consumption.
Identification of CVYV is possible by reverse transcriptase PCT (RT-PCR) and by nucleic acid hybridization, as for example described in the 2007 OEPP/EPPO Bulletin 37, pp 554-559, entitled “Cucumber vein yellowing virus (Ipomovirus)”. This document also describes mechanical and whitefly transmissions to test plants for identifying CVYV.
It is an object of the invention to provide cultivated watermelon plants, C. lanatus ssp. vulgaris, and parts of such plants, comprising a resistance locus on chromosome 3, which confers resistance against CVYV when in homozygous form in a diploid plant, or when in three or four copies in a triploid or tetraploid plant respectively. This locus is a Quantitative Trait Locus (QTL) identified in two different wild accession of Citrullus lanatus ssp. lanatus and Citrullus lanatus ssp. mucosospermus, which have white-fleshed or yellow fleshed fruits that are bitter tasting and have a very low brix (e.g. a brix of 3.0 or less), and this locus was introgressed into cultivated watermelon (Citrullus lanatus ssp. vulgaris) having marketable fruits, i.e. fruits of good quality with a brix of at least 8.0 or 9.0 or preferably at least 10.0, 11.0 or higher. This CVYV resistance-conferring QTL is herein referred to as cyv_3.1.
The cultivated watermelon plants comprising cyv_3.1 in one, two, three or four copies include diploid plants or doubled-haploid plants, tetraploid plants and triploid plants (e.g. triploid hybrid plants producing seedless fruits), as well as seeds from which these plants can be grown, and any parts of the plants comprising the cyv_3.1 (fruits, fruit parts, root stocks, scions, cells, pollen, anthers, ovules, stems, leaves, cotyledons, hypocotyls, flowers, in vitro cell or tissue cultures, in vitro propagations, etc.).
It is also an object of the invention to provide molecular markers closely linked to the CVYV resistance conferring QTL cyv_3.1 and the use of one or more of such molecular markers in a) breeding cultivated watermelon plants comprising cyv_3.1 in one (e.g. heterozygous in a diploid), two (e.g. homozygous in diploid and doubled-haploid plants), three (in a triploid) or four copies (in a tetraploid) and/or b) screening plants, plant parts, cells or genomic DNA for the presence of one or more of these molecular markers and thereby for the presence of cyv_3.1. Such breeding methods and screening methods are encompassed herein.